1. Field of the Invention
The present invention relates to an imaging lens system, and particularly relates to a compact imaging lens system suitable for use in an image pick-up device.
2. Description of Prior Art
Integration of photoelectric technology into an image pick-up device is now a trend in current technology development. To fulfill the portability requirement, the image pick-up device has to be light in weight and small in size. In the selection of imaging lens shapes and materials for use in the image pick-up device, the conventional spherical ground glass lenses have been widely adopted because there is a wide range of materials available for making such lenses and color aberration can be more easily corrected. However, the spherical ground glass lenses have the disadvantages of uneasy correction of spherical aberration and astigmatic aberration in the case of a small F-number and a large wide angle of field. To improve the above-mentioned drawbacks existed in the conventional spherical ground glass lenses, aspheric glass or plastic lenses have been introduced to help reduce these aberrations. The inclusion of aspheric lens elements in a lens system significantly increases image quality and reduces barrel distortion associated with wide-angle lenses. Further, since one aspheric lens element can replace several spherical lens elements in the lens system to perform the same aberration correction function, reduction of the amount of lens elements in the system is also enabled, which makes the lens system smaller and lighter. However, for a conventional glass lens system, to effectively correct off-axis aberrations and color aberrations associated with a wide angle of view, a relatively larger amount of lens elements must be employed. This makes such a glass lens system long in length, large in volume and high in cost, which deviates from the current compact trend. In addition, the machining of an aspheric glass lens element is very difficult, which further limits the application of such a glass lens system in a compact digital product. Comparatively, an aspheric plastic lens element is easy to machine and low in cost. Therefore, aspheric plastic lens elements are widely used in compact imaging lens systems to reduce the length of the entire system.
For a wide-angle lens system, important design considerations include correction of off-axis aberrations and color aberrations associated with a wide angle of view, serious distortions and so on. Accordingly, a wide-angle lens system design is more difficult, and various problems may arise such as unmachinability of a lens element shape and too large chief ray angles. Various wide-angle lens system designs have been disclosed in, for example, U.S. Pat. Nos. 4,493,537, 5,251,073, and 4,525,038. However, a design, which ensures effective correction of various aberrations at a very short total length while giving attention to actual machinability, is rare. Accordingly, a prism type design and a free-form surface prism type design have been proposed to reduce the total length of the lens system. The term “free-form surface” means a curved surface that is neither a plane nor part of a sphere, and the term “free-form surface prism” means a prism having a free-form surface on at least one surface. A free-form surface prism is described in, for example, U.S. Pat. No. 6,323,892. The above-mentioned prism type design folds the optical path by 90 degrees by means of a 45° reflecting mirror that has no aberration correction function.
The application of a free-form surface prism has the following advantages: (1) The total length of a lens system can be reduced by several light reflections in the free-form surface prism to obtain the light path folding effect; (2) As no chromatic aberration exists on a reflection surface of the free-form surface prism, the amount of constituting elements in the lens system can be reduced since no additional lens elements are needed to compensate a large amount of chromatic aberrations which occur when employing conventional refraction elements; (3) Assembly of the lens system is facilitated since the positional relationship between optical surfaces on the free-form surface prism are fixed. However, conventional free-form surface prisms are generally complicate in construction, which makes its design, machining and fixing difficult. Therefore, there still remains room for improvement.